Composite propellant composition of vinyl polymer with added polyisobutylene



3,424,630 CGMPOSITE PROPELLANT COMPOSITION OF VINYL POLYMER WITH ADDED POLYISOBUTYLENE Boyce M. Corley, Waco, Tern, assignor to North American Rockwell Corporation No Drawing. Filed Oct. 1, 1965, Ser. No. 492,337

US. Cl. 149-19 Int. Cl. C06!) 1/04 6 Claims This invention relates to a new improved solid propellant gas generator composition. More particularly, the invention relates to a solid propellant composition of matter having improved temperature sensitivity in gas generator grains.

The invention relates to the field of solid propellant compositions particularly useful as gas generators which have application as auxiliary power systems and starter cartridges for larger motors. The particular compositions described relative to this invention comprise an oxidizer which is ammonium nitrate and a fuel polymer binder comprised of butadiene-methyl vinylpyridine copolymer.

It is well known that propellants burn according to the following basic equation:

wherein r is the burning rate or velocity of propellant consumption in inches per second; 'P is the chamber pressure in pounds per square inch; a and n are constants. The constant a varies with the initial propellant temperature. Thus the burning rate is a function of temperature of the grain prior to combustion. The exponent n which is known as the pressure exponent is independent of the initial grain temperature but is a function of the particular propellant composition and varies depending upon the propellant. It can be seen that depending upon the value of n, the burning rate is likewise affected by the chamber pressure of the rocket motor. In order to provide a relatively pressure insensitive propellant; in other words, a propellant whose burning rate is not greatly affected by varying chamber pressures, one strives to obtain as low a value for n. as possible, yet maintaining other good ballistic characteristics for the propellant composition. Alternatively, high values of n give a rapid change of burning rate with pressure. Thus it can be appreciated that even a small change in chamber pressure can produce substantial changes in the amount of hot gas produced when the value of n is high. In gas generator applications wherein the amount of hot gas produced is the most important aspect, temperature sensitivity becomes extremely important. Since gas generators are used for driving turbines of auxiliary power devices, expelling liquids from tanks, expelling stores or emergency escape capsules from flying vehicles, the flame temperature of the propellant is appreciably lower than that of rocket propellants so the gas can be used in uncooled piping and uncooled machinery. Thus, one of the main differences between the gas generator composition and a solid rocket motor composition is that the gas generator propellant contains more fuel and less oxidizer and less energetic, usually ammonium nitrate is utilized. It has been found that the use of ammonium nitrate with a fuel binder of butadiene and methyl vinylpyridine provides particularly good gas generator compositions. How- 3,424,630 Patented Jan. 28, 1969 ever, in order to further improve such compositions, attempts have been made to obtain lower pressure exponents in the system. 'In addition to the pressure exponent and of perhaps more important is the temperature sensitivity of the propellant composition. Temperature changes affect the equilibrium chamber pressure and the burning rate in a gas generator. It has been shown that the initial temperature of the grain will material- 1y affect the performance. For example, on a hot day, a propellant will operate at a higher chamber pressure and thrust than on a cool day. As a result, the firing duration will be shorter. This indicates that the initial temperature of the grain has a decided effect on the burning rate and that temperature conditions have to be considered when exacting performance requirements are to be met. The pressure sensitivity coefiicient for a propellant is represented by the symbol 'rrK wK is defined according to the following formula:

5T KEP. (6T K As can be seen 1rK which is percent change of chamber pressure per unit of temperature change at a particular value of K, where K is the ratio of the burning surface area to the throat area. It is mathematically defined as above as the partial derivative of the natural logarithm of the equilibrium chamber pressure P with respect to temperature T. If the value of 1rK is high, the variation of equilibrium chamber pressure with temperature is likewise affected and thus will be greater. This will, of course, affect, as given in a previous formula, (1), the burning rate of the propellant causing an increase therein to result. It can thus be appreciated that the temperature sensitivity n-K can have a significant effect on the overall performance of the propellant. The lower the value, the less sensitive to temperature changes, and thus if one can lower the value of 1rK in a given propellant system without affecting other ballistic characteristics significantly, then a substantial advance in the art has been accomplished.

Thus the principal object of this invention is to provide gas generator compositions having a decreased temperature sensitivity.

Another object of this invention is to provide a solid gas generator having a decreased pressure exponent.

A further object of this invention is to provide solid gas generator compositions containing ammonium nitrate and butadiene-methyl vinylpyridine copolymer binders having improved pressure sensitivity.

The above and other objects of the invention are accomplished by the addition of polyisobutylene polymer which apparently reacts synergistically with the butadiene-methyl vinylpyridine copolymer to decrease the temperature and pressure sensitivity of the compositions. For example, the temperature sensitivity (WKN) expressed in percent over degree Farenheit from a range of -70 F. to 170 F. is reduced from 0.27 to 0.22 utilizing the polyisobutylene polymers in the propellant system. Likewise the pressure exponent n is reduced from 0.52 to as low as 0.42. It has been found that successful results have been achieved from mixtures of the butadiene-methyl vinylpyridine copolymer and polyisobutylene in a ratio of from 50 to weight percent of the polyisobutylene in the mixture which is then used to comprise the fuel binder composition of the gas generator propellant grain.

As indicated, the gas generator composition of the invention contains a predominance of ammonium nitrate as the oxidizer. Ammonium nitrate is particulate matter which is thoroughly mixed with the viscous or solid binder material of the invention. Generally, the ammonium nitrate will comprise from 70 to 80 Weight percent of the composition. Though ammonium nitrate is cited by way of specific example, other conventional solid particulate inorganic salt oxidizers such as ammonium perchlorate, potassium perchlorate and the like can be used.

In addition to the ammonium nitrate, a combustion catalyst such as Milori blue is present in an amount of 0.2 to 1.0 weight percent. Other combustion catalysts can be utilized as desired. A reinforcing agent to give physical strength to the propellant is utilized. Normally this is a carbon black because of the rubber base copolymer utilized and is normally supplied by a manufacturer together with the butadieneamethyl vinylpyridine copolymer. An example of such reinforcing or strengthening agent is Philblack A which is a form of carbon black made by the Phillips Petroleum Co. It is normally present in the amount ranging from 1 to 5 weight percent of the propel lant composition. If desired, a coolant to produce a cleaner, cooler, combustion gas may be added to the composition in an amount up to weight percent. An example of such a coolant is dicyandiamide. It is necessary that a curative to cross-link the butadiene-methyl vinylpyridine copolymer be present in the mixture. An example of such a curative is 1,4-bis(trichloro-methyl) benzene, usually in an amount equal to 10 parts per hundred rubber based on the butadiene-methyl vinylpyridine copolymer. It is required for the successful cross-linking. The remainder of the propellant composition will thus be comprised of the butadiene-methyl vinylpyridine copolymer and the polyisobutylene polymers.

The polyisobutylene polymers utilized vary in molecular range from the extent of a very viscous material to solid, relatively hard products. Examples of the polyisobutylene polymers are Vistanex LM-MS which has an average molecular weight (Staudinger) of from 8700 to 10,000. This is a relatively viscous, pasty-type material. Examples of hard granular polymer would be Vistanex L-lOO which has a molecular weight range of 81,000 to 99,000. The Vistanex polyisobutylene polymers are manufactured by the Enjay Chemical Company, Division of Humble Oil and Refining Company. Polyisobutylene polymers exist throughout the entire weight range from simple copolymers of two isobutylene monomers up to the hard, brittle products. For purposes of this invention, products that are relatively viscous are desirable since the propellant composition formed generally is to be extruded and thus cannot be too fluid. In the formulation of the propellant composition of the invention, the mixtures of various molecular weight polyisobutylene polymers can be successfully used as will be shown in the specific examples.

Example I Two separate 11 lb. batches of propellant Were made having the following compositions:

Weight percent Propellant ingredients Formulation A Formulation B Butadiene-Inethyl vinylpyridine c0- pclymer (90-10) 7.13 4. 43 Philblack A 3.02 3.19 Vistanex LM-MS (pol sobutylene polymer) 7. 13 G. 36 Vistanex L-100 (polyisobutylene (polyme 3. 65 1,4-bis(trichloromethyl) benzene 0 72 0.37 Dicyandlamide 6.00 6. 00 Milori blue 0. 50 O. 50 Ammonium nitrate 75. 50 75. 50

sion of ingredients, a composition is then placed in a 50- ton conventional extrusion press wherein the test grains are extruded. The extruded grains were 0.7 pound in weight and had dimensions of a 2.3 in. outer diameter, 1.0 in. perforation, and 3.0 in. in length. The extruded grains were then laid upon a pan and cured in an oven at 190 F. for 24 hours.

Example II Cured or uncured samples of the formulations from Example I were tested to determine the burning rate and thus calculate both the pressure exponent and temperature sensitivity. The tests are conventional ones used in the rocket propellant industry for determining such value. The small sample grains are burned in a modified Crawford strand bomb. For initial tests of uncured propellant the pressure of the bomb was 1000 p.s.i.g. Basically, the burning rate is determined from wires that are placed in various locations along the grain. As the grain burns, the wires which are connected to clocks set off at various intervals as the burning progresses to each wire, thus indicating the time it takes for the burning surface to transgress along the grain.

To particularly illustrate the improved results utilizing the polyisobutylene polymers in combination with the butadiene-methyl vinylpyridine copolymer, the following Table I shows the results of formulations A and B vs. the values obtained from a composition X equivalent to formulation A except that the binder comprises only butadiene-methyl vinylpyridine with the other components being in the same amount. As can be seen from the table both the pressure exponent is reduced significantly as well as the values of 1rK To illustrate graphically the effect of the polyisobutylene in combination with the butadiene-methyl vinylpyridine copolymer, a formulation was prepared utilizing the components in the amounts set forth in formulation B except that the binder com-prised 100 percent of the polyisobutylene LMF-MS. Results from this formulation disclose that a value of n'K equivalent to 0.68 was obtained, clearly showing that the polyisobutylene by itself produces very poor temperature sensitivity.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. A gas generator composition comprising:

a solid particulate inorganic solid oxidizer salt,

and a fuel-binder of butadiene-methyl vinylpyridine copolymer having 50 to weight percent of polyisobutylene mixed therewith.

2. The composition of claim 1 wherein said oxidizer is ammonium nitrate.

3. The composition of claim 1 wherein said oxidizer comprises 70 to 80 weight percent of the gas generator composition.

4. A method of decreasing the temperature sensitivity and pressure exponent of gas generator compositions having a solid particulate inorganic salt oxidizer in a fuelbinder of butadiene-methyl vinylpyridine copolymer comprising adding to said binder polyisobutylene.

5. The method of claim 4 wherein the polyisobutylene is added in the amount of 50 to 80 weight percent of the binder composition.

6. A binder for solid propellant gas generators comprising:

20 to 50 Weight percent butadiene-methyl vinylpyridine copolymer and 50 to 80 weight percent polyisobutylene.

References Cited UNITED STATES PATENTS 2,994,598 8/1961 Dickey 14919 Downard 14919 Ratliff et a1 149-19 Corley et a1. 14919 Ratliff 149-19 US. Cl. X.R. 

